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PERLFAQ5
!!!PERLFAQ5
NAME
DESCRIPTION
AUTHOR AND COPYRIGHT
----
!!NAME


perlfaq5 - Files and Formats ($Revision: 1.38 $, $Date: 1999/05/23 16:08:30 $)
!!DESCRIPTION


This section deals with I/O and the ``f'' issues:
filehandles, flushing, formats, and footers.


__How do I flush/unbuffer an output filehandle? Why must I
do this?__


The C standard I/O library (stdio) normally buffers
characters sent to devices. This is done for efficiency
reasons so that there isn't a system call for each byte. Any
time you use ''print()'' or ''write()'' in Perl, you
go though this buffering. ''syswrite()'' circumvents
stdio and buffering.


In most stdio implementations, the type of output buffering
and the size of the buffer varies according to the type of
device. Disk files are block buffered, often with a buffer
size of more than 2k. Pipes and sockets are often buffered
with a buffer size between 1/2 and 2k. Serial devices (e.g.
modems, terminals) are normally line-buffered, and stdio
sends the entire line when it gets the newline.


Perl does not support truly unbuffered output (except
insofar as you can syswrite(OUT, $char, 1)). What
it does instead support is ``command buffering'', in which a
physical write is performed after every output command. This
isn't as hard on your system as unbuffering, but does get
the output where you want it when you want it.


If you expect characters to get to your device when you
print them there, you'll want to autoflush its handle. Use
''select()'' and the $ variable to control
autoflushing (see perlvar/$ and ``select'' in
perlfunc):


    $old_fh = select(OUTPUT_HANDLE);
$ = 1;
select($old_fh);
Or using the traditional idiom:


    select((select(OUTPUT_HANDLE), $ = 1)[[0]);
Or if don't mind slowly loading several thousand lines of module code just because you're afraid of the $ variable:


    use !FileHandle;
open(DEV,
or the newer IO::* modules:


    use IO::Handle;
open(DEV,
or even this:


    use IO::Socket;               # this one is kinda a pipe?
$sock = IO::Socket::INET-
    $sock-
Note the bizarrely hardcoded carriage return and newline in their octal equivalents. This is the ONLY way (currently) to assure a proper flush on all platforms, including Macintosh. That's the way things work in network programming: you really should specify the exact bit pattern on the network line terminator. In practice,  often works, but this is not portable.


See perlfaq9 for other examples of fetching URLs over the
web.


__How do I change one line in a file/delete a line in a
file/insert a line in the middle of a file/append to the
beginning of a file?__


Those are operations of a text editor. Perl is not a text
editor. Perl is a programming language. You have to
decompose the problem into low-level calls to read, write,
open, close, and seek.


Although humans have an easy time thinking of a text file as
being a sequence of lines that operates much like a stack of
playing cards--or punch cards--computers usually see the
text file as a sequence of bytes. In general, there's no
direct way for Perl to seek to a particular line of a file,
insert text into a file, or remove text from a
file.


(There are exceptions in special circumstances. You can add
or remove data at the very end of the file. A sequence of
bytes can be replaced with another sequence of the same
length. The $DB_RECNO array bindings as documented
in DB_File also provide a direct way of modifying a file.
Files where all lines are the same length are also easy to
alter.)


The general solution is to create a temporary copy of the
text file with the changes you want, then copy that over the
original. This assumes no locking.


    $old = $file;
$new =
    open(OLD,
    # Correct typos, preserving case
while (
    close(OLD)                  or die
    rename($old, $bak)          or die
Perl can do this sort of thing for you automatically with the -i command-line switch or the closely-related $^I variable (see perlrun for more details). Note that -i may require a suffix on some non-Unix systems; see the platform-specific documentation that came with your port.


    # Renumber a series of tests from the command line
perl -pi -e 's/(^s+tests+)d+/ $1 . ++$count /e' t/op/taint.t
    # form a script
local($^I, @ARGV) = ('.orig', glob(
If you need to seek to an arbitrary line of a file that changes infrequently, you could build up an index of byte positions of where the line ends are in the file. If the file is large, an index of every tenth or hundredth line end would allow you to seek and read fairly efficiently. If the file is sorted, try the look.pl library (part of the standard perl distribution).


In the unique case of deleting lines at the end of a file,
you can use ''tell()'' and ''truncate()''. The
following code snippet deletes the last line of a file
without making a copy or reading the whole file into
memory:


        open (FH,
Error checking is left as an exercise for the reader.


__How do I count the number of lines in a
file?__


One fairly efficient way is to count newlines in the file.
The following program uses a feature of tr///, as documented
in perlop. If your text file doesn't end with a newline,
then it's not really a proper text file, so this may report
one fewer line than you expect.


    $lines = 0;
open(FILE, $filename) or die
This assumes no funny games with newline translations.


__How do I make a temporary file name?__


Use the new_tmpfile class method from the IO::File
module to get a filehandle opened for reading and writing.
Use it if you don't need to know the file's
name:


    use IO::File;
$fh = IO::File-
If you do need to know the file's name, you can use the tmpnam function from the POSIX module to get a filename that you then open yourself:


    use Fcntl;
use POSIX qw(tmpnam);
    # try new temporary filenames until we get one that didn't already
# exist;  the check should be unnecessary, but you can't be too careful
do { $name = tmpnam() }
until sysopen(FH, $name, O_RDWRO_CREATO_EXCL);
    # install atexit-style handler so that when we exit or die,
# we automatically delete this temporary file
END { unlink($name) or die
    # now go on to use the file ...
If you're committed to creating a temporary file by hand, use the process ID and/or the current time-value. If you need to have many temporary files in one process, use a counter:


    BEGIN {
use Fcntl;
my $temp_dir = -d '/tmp' ? '/tmp' : $ENV{TMP}  $ENV{TEMP};
my $base_name = sprintf(


__How can I manipulate fixed-record-length
files?__


The most efficient way is using ''pack()'' and
''unpack()''. This is faster than using ''substr()''
when taking many, many strings. It is slower for just a
few.


Here is a sample chunk of code to break up and put back
together again some fixed-format input lines, in this case
from the output of a normal, Berkeley-style ps:


    # sample input line:
#   15158 p5  T      0:00 perl /home/tchrist/scripts/now-what
$PS_T = 'A6 A4 A7 A5 A*';
open(PS,
We've used $$var in a way that forbidden by use strict 'refs'. That is, we've promoted a string to a scalar variable reference using symbolic references. This is ok in small programs, but doesn't scale well. It also only works on global variables, not lexicals.


__How can I make a filehandle local to a subroutine? How do
I pass filehandles between subroutines? How do I make an
array of filehandles?__


The fastest, simplest, and most direct way is to localize
the typeglob of the filehandle in question:


    local *!TmpHandle;
Typeglobs are fast (especially compared with the alternatives) and reasonably easy to use, but they also have one subtle drawback. If you had, for example, a function named ''!TmpHandle()'', or a variable named %!TmpHandle, you just hid it from yourself.


    sub findme {
local *!HostFile;
open(!HostFile,
Here's how to use typeglobs in a loop to open and store a bunch of filehandles. We'll use as values of the hash an ordered pair to make it easy to sort the hash in insertion order.


    @names = qw(motd termcap passwd hosts);
my $i = 0;
foreach $filename (@names) {
local *FH;
open(FH,
    # Using the filehandles in the array
foreach $name (sort { $file{$a}[[0]
For passing filehandles to functions, the easiest way is to preface them with a star, as in func(*STDIN). See ``Passing Filehandles'' in perlfaq7 for details.


If you want to create many anonymous handles, you should
check out the Symbol, !FileHandle, or IO::Handle (etc.)
modules. Here's the equivalent code with Symbol::gensym,
which is reasonably light-weight:


    foreach $filename (@names) {
use Symbol;
my $fh = gensym();
open($fh,
Here's using the semi-object-oriented !FileHandle module, which certainly isn't light-weight:


    use !FileHandle;
    foreach $filename (@names) {
my $fh = !FileHandle-
Please understand that whether the filehandle happens to be a (probably localized) typeglob or an anonymous handle from one of the modules in no way affects the bizarre rules for managing indirect handles. See the next question.


__How can I use a filehandle indirectly?__


An indirect filehandle is using something other than a
symbol in a place that a filehandle is expected. Here are
ways to get indirect filehandles:


    $fh =   SOME_FH;       # bareword is strict-subs hostile
$fh =
Or, you can use the new method from the !FileHandle or IO modules to create an anonymous filehandle, store that in a scalar variable, and use it as though it were a normal filehandle.


    use !FileHandle;
$fh = !FileHandle-
    use IO::Handle;                     # 5.004 or higher
$fh = IO::Handle-
Then use any of those as you would a normal filehandle. Anywhere that Perl is expecting a filehandle, an indirect filehandle may be used instead. An indirect filehandle is just a scalar variable that contains a filehandle. Functions like print, open, seek, or the  diamond operator will accept either a read filehandle or a scalar variable containing one:


    ($ifh, $ofh, $efh) = (*STDIN, *STDOUT, *STDERR);
print $ofh
If you're passing a filehandle to a function, you can write the function in two ways:


    sub accept_fh {
my $fh = shift;
print $fh
Or it can localize a typeglob and use the filehandle directly:


    sub accept_fh {
local *FH = shift;
print  FH
Both styles work with either objects or typeglobs of real filehandles. (They might also work with strings under some circumstances, but this is risky.)


    accept_fh(*STDOUT);
accept_fh($handle);
In the examples above, we assigned the filehandle to a scalar variable before using it. That is because only simple scalar variables, not expressions or subscripts of hashes or arrays, can be used with built-ins like print, printf, or the diamond operator. Using something other than a simple scalar varaible as a filehandle is illegal and won't even compile:


    @fd = (*STDIN, *STDOUT, *STDERR);
print $fd[[1]
With print and printf, you get around this by using a block and an expression where you would place the filehandle:


    print  { $fd[[1] }
That block is a proper block like any other, so you can put more complicated code there. This sends the message out to one of two places:


    $ok = -x
This approach of treating print and printf like object methods calls doesn't work for the diamond operator. That's because it's a real operator, not just a function with a comma-less argument. Assuming you've been storing typeglobs in your structure as we did above, you can use the built-in function named readline to reads a record just as  does. Given the initialization shown above for @fd, this would work, but only because ''readline()'' require a typeglob. It doesn't work with objects or strings, which might be a bug we haven't fixed yet.


    $got = readline($fd[[0]);
Let it be noted that the flakiness of indirect filehandles is not related to whether they're strings, typeglobs, objects, or anything else. It's the syntax of the fundamental operators. Playing the object game doesn't help you at all here.


__How can I set up a footer format to be used with__
''write()''__?__


There's no builtin way to do this, but perlform has a couple
of techniques to make it possible for the intrepid
hacker.


__How can I__ ''write()'' __into a
string?__


See ``Accessing Formatting Internals'' in perlform for an
''swrite()'' function.


__How can I output my numbers with commas
added?__


This one will do it for you:


    sub commify {
local $_  = shift;
1 while s/^([[-+]?d+)(d{3})/$1,$2/;
return $_;
}
    $n = 23659019423.2331;
print
    GOT: 23,659,019,423.2331
You can't just:


    s/^([[-+]?d+)(d{3})/$1,$2/g;
because you have to put the comma in and then recalculate your position.


Alternatively, this code commifies all numbers in a line
regardless of whether they have decimal portions, are
preceded by + or -, or whatever:


    # from Andrew Johnson


__How can I translate tildes (~) in a
filename?__


Use the glob()'') operator, documented in
perlfunc. Older versions of Perl require that you have a
shell installed that groks tildes. Recent perl versions have
this feature built in. The Glob::KGlob module (available
from CPAN ) gives more portable glob
functionality.


Within Perl, you may use this directly:


        $filename =~ s{
^ ~             # find a leading tilde
(               # save this in $1
[[^/]        # a non-slash character
*     # repeated 0 or more times (0 means me)
)
}{
$1
? (getpwnam($1))[[7]
: ( $ENV{HOME}  $ENV{LOGDIR} )
}ex;


__How come when I open a file read-write it wipes it
out?__


Because you're using something like this, which truncates
the file and ''then'' gives you read-write
access:


    open(FH,
Whoops. You should instead use this, which will fail if the file doesn't exist.


    open(FH,
Using ``


Here are examples of many kinds of file opens. Those using
''sysopen()'' all assume


    use Fcntl;
To open file for reading:


    open(FH,
To open file for writing, create new file if needed or else truncate old file:


    open(FH,
To open file for writing, create new file, file must not exist:


    sysopen(FH, $path, O_WRONLYO_EXCLO_CREAT)          die $!;
sysopen(FH, $path, O_WRONLYO_EXCLO_CREAT, 0666)    die $!;
To open file for appending, create if necessary:


    open(FH,
To open file for appending, file must exist:


    sysopen(FH, $path, O_WRONLYO_APPEND)                die $!;
To open file for update, file must exist:


    open(FH,
To open file for update, create file if necessary:


    sysopen(FH, $path, O_RDWRO_CREAT)                   die $!;
sysopen(FH, $path, O_RDWRO_CREAT, 0666)             die $!;
To open file for update, file must not exist:


    sysopen(FH, $path, O_RDWRO_EXCLO_CREAT)            die $!;
sysopen(FH, $path, O_RDWRO_EXCLO_CREAT, 0666)      die $!;
To open a file without blocking, creating if necessary:


    sysopen(FH,
Be warned that neither creation nor deletion of files is guaranteed to be an atomic operation over NFS . That is, two processes might both successfully create or unlink the same file! Therefore O_EXCL isn't as exclusive as you might wish.


See also the new perlopentut if you have it (new for
5.6).


__Why do I sometimes get an ``Argument list too long'' when
I use __


The  operator performs a globbing operation
(see above). In Perl versions earlier than v5.6.0, the
internal ''glob()'' operator forks csh(1) to do
the actual glob expansion, but csh can't handle more than
127 items and so gives the error message Argument list
too long. People who installed tcsh as csh won't have
this problem, but their users may be surprised by
it.


To get around this, either upgrade to Perl v5.6.0 or later,
do the glob yourself with ''readdir()'' and patterns, or
use a module like Glob::KGlob, one that doesn't use the
shell to do globbing.


__Is there a leak/bug in__
''glob()''__?__


Due to the current implementation on some operating systems,
when you use the ''glob()'' function or its angle-bracket
alias in a scalar context, you may cause a memory leak
and/or unpredictable behavior. It's best therefore to use
''glob()'' only in list context.


__How can I open a file with a leading ``
__


Normally perl ignores trailing blanks in filenames, and
interprets certain leading characters (or a trailing ``'')
to mean something special. To avoid this, you might want to
use a routine like the one below. It turns incomplete
pathnames into explicit relative ones, and tacks a trailing
null byte on the name to make perl leave it
alone:


    sub safe_filename {
local $_  = shift;
s#^([[^./])#./$1#;
$_ .=
    $badpath =
This assumes that you are using POSIX (portable operating systems interface) paths. If you are on a closed, non-portable, proprietary system, you may have to adjust the  above.


It would be a lot clearer to use ''sysopen()'',
though:


    use Fcntl;
$badpath =
For more information, see also the new perlopentut if you have it (new for 5.6).


__How can I reliably rename a file?__


Well, usually you just use Perl's ''rename()'' function.
That may not work everywhere, though, particularly when
renaming files across file systems. Some sub-Unix systems
have broken ports that corrupt the semantics of
''rename()''--for example, WinNT does this right, but
Win95 and Win98 are broken. (The last two parts are not
surprising, but the first is. :-)


If your operating system supports a proper mv(1)
program or its moral equivalent, this works:


    rename($old, $new) or system(
It may be more compelling to use the File::Copy module instead. You just copy to the new file to the new name (checking return values), then delete the old one. This isn't really the same semantically as a real ''rename()'', though, which preserves metainformation like permissions, timestamps, inode info, etc.


Newer versions of File::Copy exports a ''move()''
function.


__How can I lock a file?__


Perl's builtin ''flock()'' function (see perlfunc for
details) will call flock(2) if that exists,
fcntl(2) if it doesn't (on perl version 5.004 and
later), and lockf(3) if neither of the two previous
system calls exists. On some systems, it may even use a
different form of native locking. Here are some gotchas with
Perl's ''flock()'':


1


Produces a fatal error if none of the three system calls (or
their close equivalent) exists.


2


lockf(3) does not provide shared locking, and
requires that the filehandle be open for writing (or
appending, or read/writing).


3


Some versions of ''flock()'' can't lock files over a
network (e.g. on NFS file systems), so you'd
need to force the use of fcntl(2) when you build
Perl. But even this is dubious at best. See the flock entry
of perlfunc and the ''INSTALL'' file in
the source distribution for information on building Perl to
do this.


Two potentially non-obvious but traditional flock semantics
are that it waits indefinitely until the lock is granted,
and that its locks are ''merely advisory''. Such
discretionary locks are more flexible, but offer fewer
guarantees. This means that files locked with ''flock()''
may be modified by programs that do not also use
''flock()''. Cars that stop for red lights get on well
with each other, but not with cars that don't stop for red
lights. See the perlport manpage, your port's specific
documentation, or your system-specific local manpages for
details. It's best to assume traditional behavior if you're
writing portable programs. (If you're not, you should as
always feel perfectly free to write for your own system's
idiosyncrasies (sometimes called ``features''). Slavish
adherence to portability concerns shouldn't get in the way
of your getting your job done.)


For more information on file locking, see also ``File
Locking'' in perlopentut if you have it (new for
5.6).


__Why can't I just open( FH ,
``


A common bit of code __NOT TO USE__ is
this:


    sleep(3) while -e
This is a classic race condition: you take two steps to do something which must be done in one. That's why computer hardware provides an atomic test-and-set instruction. In theory, this ``ought'' to work:


    sysopen(FH,
except that lamentably, file creation (and deletion) is not atomic over NFS , so this won't work (at least, not every time) over the net. Various schemes involving ''link()'' have been suggested, but these tend to involve busy-wait, which is also subdesirable.


__I still don't get locking. I just want to increment the
number in the file. How can I do this?__


Didn't anyone ever tell you web-page hit counters were
useless? They don't count number of hits, they're a waste of
time, and they serve only to stroke the writer's vanity.
It's better to pick a random number; they're more
realistic.


Anyway, this is what you can do if you can't help
yourself.


    use Fcntl qw(:DEFAULT :flock);
sysopen(FH,
Here's a much better web-page hit counter:


    $hits = int( (time() - 850_000_000) / rand(1_000) );
If the count doesn't impress your friends, then the code might. :-)


__How do I randomly update a binary file?__


If you're just trying to patch a binary, in many cases
something as simple as this works:


    perl -i -pe 's{window manager}{window mangler}g' /usr/bin/emacs
However, if you have fixed sized records, then you might do something more like this:


    $RECSIZE = 220; # size of record, in bytes
$recno   = 37;  # which record to update
open(FH,
Locking and error checking are left as an exercise for the reader. Don't forget them or you'll be quite sorry.


__How do I get a file's timestamp in perl?__


If you want to retrieve the time at which the file was last
read, written, or had its meta-data (owner, etc) changed,
you use the __-M__, __-A__, or __-C__ filetest
operations as documented in perlfunc. These retrieve the age
of the file (measured against the start-time of your
program) in days as a floating point number. To retrieve the
``raw'' time in seconds since the epoch, you would call the
stat function, then use ''localtime()'', ''gmtime()'',
or ''POSIX::strftime()'' to convert this into
human-readable form.


Here's an example:


    $write_secs = (stat($file))[[9];
printf
If you prefer something more legible, use the File::stat module (part of the standard distribution in version 5.004 and later):


    # error checking left as an exercise for reader.
use File::stat;
use Time::localtime;
$date_string = ctime(stat($file)-
The ''POSIX::strftime()'' approach has the benefit of being, in theory, independent of the current locale. See perllocale for details.


__How do I set a file's timestamp in perl?__


You use the ''utime()'' function documented in ``utime''
in perlfunc. By way of example, here's a little program that
copies the read and write times from its first argument to
all the rest of them.


    if (@ARGV
Error checking is, as usual, left as an exercise for the reader.


Note that ''utime()'' currently doesn't work correctly
with Win95/NT ports. A bug has been reported. Check it
carefully before using ''utime()'' on those
platforms.


__How do I print to more than one file at
once?__


If you only have to do this once, you can do
this:


    for $fh (FH1, FH2, FH3) { print $fh
To connect up to one filehandle to several output filehandles, it's easiest to use the tee(1) program if you have it, and let it take care of the multiplexing:


    open (FH,
Or even:


    # make STDOUT go to three files, plus original STDOUT
open (STDOUT,
Otherwise you'll have to write your own multiplexing print function--or your own tee program--or use Tom Christiansen's, at http://www.perl.com/CPAN/authors/id/TOMC/scripts/tct.gz , which is written in Perl and offers much greater functionality than the stock version.


__How can I read in an entire file all at
once?__


The customary Perl approach for processing all the lines in
a file is to do so one line at a time:


    open (INPUT, $file)          die
This is tremendously more efficient than reading the entire file into memory as an array of lines and then processing it one element at a time, which is often--if not almost always--the wrong approach. Whenever you see someone do this:


    @lines =
you should think long and hard about why you need everything loaded at once. It's just not a scalable solution. You might also find it more fun to use the standard DB_File module's $DB_RECNO bindings, which allow you to tie an array to a file so that accessing an element the array actually accesses the corresponding line in the file.


On very rare occasion, you may have an algorithm that
demands that the entire file be in memory at once as one
scalar. The simplest solution to that is


    $var = `cat $file`;
Being in scalar context, you get the whole thing. In list context, you'd get a list of all the lines:


    @lines = `cat $file`;
This tiny but expedient solution is neat, clean, and portable to all systems on which decent tools have been installed. For those who prefer not to use the toolbox, you can of course read the file manually, although this makes for more complicated code.


    {
local(*INPUT, $/);
open (INPUT, $file)      die
That temporarily undefs your record separator, and will automatically close the file at block exit. If the file is already open, just use this:


    $var = do { local $/;


__How can I read in a file by paragraphs?__


Use the $/ variable (see perlvar for details). You
can either set it to  to eliminate
empty paragraphs (, for
instance, gets treated as two paragraphs and not three), or
 to accept empty
paragraphs.


Note that a blank line must have no blanks in it. Thus
 is one paragraph, but
 is two.


__How can I read a single character from a file? From the
keyboard?__


You can use the builtin getc() function for most
filehandles, but it won't (easily) work on a terminal
device. For STDIN , either use the
Term::!ReadKey module from CPAN or use the
sample code in ``getc'' in perlfunc.


If your system supports the portable operating system
programming interface ( POSIX ), you can use
the following code, which you'll note turns off echo
processing as well.


    #!/usr/bin/perl -w
use strict;
$ = 1;
for (1..4) {
my $got;
print
    BEGIN {
use POSIX qw(:termios_h);
        my ($term, $oterm, $echo, $noecho, $fd_stdin);
        $fd_stdin = fileno(STDIN);
        $term     = POSIX::Termios-
        $echo     = ECHO  ECHOK  ICANON;
$noecho   = $oterm
        sub cbreak {
$term-
        sub cooked {
$term-
        sub getone {
my $key = '';
cbreak();
sysread(STDIN, $key, 1);
cooked();
return $key;
}
    }
    END { cooked() }
The Term::!ReadKey module from CPAN may be easier to use. Recent versions include also support for non-portable systems as well.


    use Term::!ReadKey;
open(TTY,
For legacy DOS systems, Dan Carson


To put the PC in ``raw'' mode, use ioctl with
some magic numbers gleaned from msdos.c (Perl source file)
and Ralf Brown's interrupt list (comes across the net every
so often):


    $old_ioctl = ioctl(STDIN,0,0);     # Gets device info
$old_ioctl
Then to read a single character:


    sysread(STDIN,$c,1);               # Read a single character
And to put the PC back to ``cooked'' mode:


    ioctl(STDIN,1,$old_ioctl);         # Sets it back to cooked mode.
So now you have $c. If ord($c) == 0, you have a two byte code, which means you hit a special key. Read another byte with sysread(STDIN,$c,1), and that value tells you what combination it was according to this table:


    # PC 2-byte keycodes = ^@ + the following:
    # HEX     KEYS
# ---     ----
# 0F      SHF TAB
# 10-19   ALT QWERTYUIOP
# 1E-26   ALT ASDFGHJKL
# 2C-32   ALT ZXCVBNM
# 3B-44   F1-F10
# 47-49   HOME,UP,!PgUp
# 4B      LEFT
# 4D      RIGHT
# 4F-53   END,DOWN,!PgDn,Ins,Del
# 54-5D   SHF F1-F10
# 5E-67   CTR F1-F10
# 68-71   ALT F1-F10
# 73-77   CTR LEFT,RIGHT,END,!PgDn,HOME
# 78-83   ALT 1234567890-=
# 84      CTR !PgUp
This is all trial and error I did a long time ago; I hope I'm reading the file that worked...


__How can I tell whether there's a character waiting on a
filehandle?__


The very first thing you should do is look into getting the
Term::!ReadKey extension from CPAN . As we
mentioned earlier, it now even has limited support for
non-portable (read: not open systems, closed, proprietary,
not POSIX , not Unix, etc)
systems.


You should also check out the Frequently Asked Questions
list in comp.unix.* for things like this: the answer is
essentially the same. It's very system dependent. Here's one
solution that works on BSD
systems:


    sub key_ready {
my($rin, $nfd);
vec($rin, fileno(STDIN), 1) = 1;
return $nfd = select($rin,undef,undef,0);
}
If you want to find out how many characters are waiting, there's also the FIONREAD ioctl call to be looked at. The ''h2ph'' tool that comes with Perl tries to convert C include files to Perl code, which can be required. FIONREAD ends up defined as a function in the ''sys/ioctl.ph'' file:


    require 'sys/ioctl.ph';
    $size = pack(
If ''h2ph'' wasn't installed or doesn't work for you, you can ''grep'' the include files by hand:


    % grep FIONREAD /usr/include/*/*
/usr/include/asm/ioctls.h:#define FIONREAD      0x541B
Or write a small C program using the editor of champions:


    % cat
And then hard-code it, leaving porting as an exercise to your successor.


    $FIONREAD = 0x4004667f;         # XXX: opsys dependent
    $size = pack(
FIONREAD requires a filehandle connected to a stream, meaning that sockets, pipes, and tty devices work, but ''not'' files.


__How do I do a__ tail -f __in
perl?__


First try


    seek(GWFILE, 0, 1);
The statement seek(GWFILE, 0, 1) doesn't change the current position, but it does clear the end-of-file condition on the handle, so that the next GWFILE


If that doesn't work (it relies on features of your stdio
implementation), then you need something more like
this:


        for (;;) {
for ($curpos = tell(GWFILE);
If this still doesn't work, look into the POSIX module. POSIX defines the ''clearerr()'' method, which can remove the end of file condition on a filehandle. The method: read until end of file, ''clearerr()'', read some more. Lather, rinse, repeat.


There's also a File::Tail module from CPAN
.


__How do I__ ''dup()'' __a filehandle in
Perl?__


If you check ``open'' in perlfunc, you'll see that several
of the ways to call ''open()'' should do the trick. For
example:


    open(LOG,
Or even with a literal numeric descriptor:


   $fd = $ENV{MHCONTEXTFD};
open(MHCONTEXT,
Note that ``


Error checking, as always, has been left as an exercise for
the reader.


__How do I close a file descriptor by
number?__


This should rarely be necessary, as the Perl ''close()''
function is to be used for things that Perl opened itself,
even if it was a dup of a numeric descriptor as with
MHCONTEXT above. But if you really have to,
you may be able to do this:


    require 'sys/syscall.ph';
$rc = syscall(
Or, just use the fdopen(3S) feature of ''open()'':


    {
local *F;
open F,


__Why can't I use ``C:tempfoo'' in DOS
paths? What doesn't `C:tempfoo.exe` work?__


Whoops! You just put a tab and a formfeed into that
filename! Remember that within double quoted strings
(``likethis''), the backslash is an escape character. The
full list of these is in ``Quote and Quote-like Operators''
in perlop. Unsurprisingly, you don't have a file called
``c:(tab)emp(formfeed)oo'' or ``c:(tab)emp(formfeed)oo.exe''
on your legacy DOS filesystem.


Either single-quote your strings, or (preferably) use
forward slashes. Since all DOS and Windows
versions since something like MS-DOS 2.0 or so have treated
/ and \ the same in a path, you might as
well use the one that doesn't clash with Perl--or the
POSIX shell, ANSI C and C
++ , awk, Tcl, Java, or Python, just to
mention a few. POSIX paths are more portable,
too.


__Why doesn't glob(``*.*'') get all the
files?__


Because even on non-Unix ports, Perl's glob function follows
standard Unix globbing semantics. You'll need
glob( to get all (non-hidden) files.
This makes ''glob()'' portable even to legacy systems.
Your port may include proprietary globbing functions as
well. Check its documentation for details.


__Why does Perl let me delete read-only files? Why does__
-i __clobber protected files? Isn't this a bug in
Perl?__


This is elaborately and painstakingly described in the ``Far
More Than You Ever Wanted To Know'' in
http://www.perl.com/CPAN/doc/FMTEYEWTK/file-dir-perms
.


The executive summary: learn how your filesystem works. The
permissions on a file say what can happen to the data in
that file. The permissions on a directory say what can
happen to the list of files in that directory. If you delete
a file, you're removing its name from the directory (so the
operation depends on the permissions of the directory, not
of the file). If you try to write to the file, the
permissions of the file govern whether you're allowed
to.


__How do I select a random line from a
file?__


Here's an algorithm from the Camel Book:


    srand;
rand($.)
This has a significant advantage in space over reading the whole file in. A simple proof by induction is available upon request if you doubt the algorithm's correctness.


__Why do I get weird spaces when I print an array of
lines?__


Saying


    print
joins together the elements of @lines with a space between them. If @lines were ( then the above statement would print


    little fluffy clouds
but if each element of @lines was a line of text, ending a newline character ( then it would print:


    little
fluffy
clouds
If your array contains lines, just print them:


    print @lines;
!!AUTHOR AND COPYRIGHT


Copyright (c) 1997-1999 Tom Christiansen and Nathan
Torkington. All rights reserved.


When included as an integrated part of the Standard
Distribution of Perl or of its documentation (printed or
otherwise), this works is covered under Perl's Artistic
License. For separate distributions of all or part of this
FAQ outside of that, see
perlfaq.


Irrespective of its distribution, all code examples here are
in the public domain. You are permitted and encouraged to
use this code and any derivatives thereof in your own
programs for fun or for profit as you see fit. A simple
comment in the code giving credit to the FAQ
would be courteous but is not required.
----
One page links to perlfaq5(1):
Man1p
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Last edited on Tuesday, June 4, 2002 12:22:34 am by "perry"
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